Doesn't matter in terms of scaling. For UX / UI you can build bad and good ui/ux with both it's rather what you build with it, both have the needed building blocks. Pick whatever you feel more comfortable with. If you feel like you will have a lot of unique UI components with unique behaviour a component library like shadcn might be a little bit better. But this is a 0.5% decision not worth wastin to much time. Pick whatever you are more comfortable with as a programmer imo